The present invention relates to improved green strength of unvulcanized elastomers. More specifically, the present invention relates to the improvement of green strength by adding semi-crystalline butene polymers to elastomers. Science and technology in the elastomer field have improved to such an extent that synthetic elastomers have supplemented or replaced natural rubber to a great extent in the fabrication of tires and other rubber products. Stereo-regular polymers and particularly synthetic high cis-1,4-polyisoprene have demonstrated physical properties similar to natural rubber and thus are capable of becoming a complete replacement for it. A major deficiency of many synthetic elastomers including cis-1,4-polyisoprene is their lack of sufficient green strength required for satisfactory fabrication of tires and industrial goods. The abatement of this deficiency has long been sought by the art and would greatly facilitate in the replacement of natural rubber which is solely produced in tropical climates.
The term "green strength" while being commonly employed and generally understood by persons skilled in the rubber industry, is nevertheless a difficult property to precisely define. Basically, it is that property of a polymer, common in natural rubber, which contributes the proper building characteristics where multiple components are employed and which result in little or no relative movement of the assembled components subsequent to assembly and prior to initiation of the curing operation. "Tack" is also an important property but the lack of tack is usually readily overcome by the addition of well known and conventional tackifying agents. Thus, green strength, that is adequate mechanical strength for fabricating operations necessarily carried out prior to vulcanization with synthetic homopolymers or interpolymers, is lacking. That is, generally the maximum or "yield" stress which the unvulcanized compositions will exhibit during deformation is rather low and moreover, the stress drops off quite rapidly as the deformation continues. Thus, unvulcanized strips or other forms of the elastomer often pull apart in a taffy-like manner during building operations. Although numerous additives and compounds have been utilized in association with various elastomers and particularly synthetic cis-1,4-polyisoprene, adequate improvement in green strength has generally not been accomplished.
Green strength has generally been measured by stress/strain curves of unvulcanized compounds. Usually, the performance of a green compound is based upon two points of the stress/strain curve, namely the first peak or yield point and the ultimate or breaking tensile. Improvement in either of these stress properties indicates improved green strength.
Among the various additive compounds or agents which have been utilized to improve green strength of synthetic elastomers are numerous nitroso compounds as set forth in U.S. Pat. Nos. 2,457,331; 2,447,015; 2,518,576; 2,526,504; 2,540,596; 2,690,780; and 3,093,614. Additionally, various dioxime compounds have been utilized such as those set forth in U.S. Pat. Nos. 2,969,341; 3,037,954; 3,160,595; and British Pat. No. 896,309. Yet another class of additives or compounds is the diesters of 5-norbornene as set forth in U.S. Pat. Nos. 3,817,883 and 3,843,613.
A Romanian article was published in Materiale Plastice No. 10 (11), 604-607 (1973) entitled "INFLUENCE OF ADDITIONS OF POLYBUTYLENE WITH DIFFERENT MOLECULAR WEIGHT ON THE PROPERTIES OF COMPOSITIONS OF CIS-POLYISOPRENE SYNTHETIC RUBBER" and prepared by B. Mehr and T. Volintiru. This article discloses the use of polybutylene which is mixed with synthetic or natural rubber to give increases in various properties. As set forth on Page 2 of the translation of the article, the molecular weight must be low and cannot exceed 10,000 since otherwise a mixture with a synthetic rubber cannot be obtained. The article does set forth data which shows that various physical properties are improved such as the increase in resistance to repeated bending and increased ozone resistance. However, as plainly evident by the table set forth on Page 3 of the translation, no appreciable increase in tensile or rupture strength is obtained when polybutylene is utilized. In fact, the bottom of Page 3 of the translation clearly states that in comparison with low pressure polyethylene utilized as a mixture, the polybutylenes do not improve the green strength of cis-polyisoprene synthetic rubber. Thus, this article is not pertinent.
Another U.S. Patent, namely U.S. Pat. No. 3,909,463 assigned to Allied Chemical Corporation, relates to the preparation of graft copolymers wherein a synthetic rubber is grafted onto an olefin polymer backbone such as polypropylene and polybutylene whereby composition is formed which is free of substantial amounts of cross-linked rubber. The amount of olefin polymer utilized is from 40 percent to about 99 percent by weight. Additionally, a bifunctional phenol-aldehyde condensate is utilized in forming the graft copolymer. The grafted copolymer formed appears to be a high impact resin.
The present application is readily distinguished from the above reference in that a physical blend is formed as opposed to a chemical reaction for forming a graft copolymer, a rubber compound is formed as opposed to a high impact resin, low amounts of polybutylene are utilized in comparison to the high amount contained in the graft copolymer and no bifunctional phenol-aldehyde condensate is utilized whatsoever in the present invention.